Fixed focus cameras are provided with a relatively wide-angle photographic lens of which effective aperture is confined to a small size by a stop, so that the photographic lens is preset to have such a large depth of field that allows to omit focusing. Because the fixed focus cameras do not need any focusing mechanism, their structures can be so simple as to save the manufacturing cost.
Lens-fitted photo film units produced by the present applicant are a kind of fixed focus camera. The lens-fitted photo film unit is reloaded with a roll of unexposed photo filmstrip, so that anyone can take photographs of satisfactory quality just by winding the film and making shutter release operations.
The lens-fitted photo film unit has a photographic lens that consists of at least a lens element made of optical plastics, for example, a couple of positive lenses. Where the contained filmstrip has a film speed of ISO 400, the lens-fitted photo film unit regularly has a fixed stop having an f-number of 11 to 10, and a fixed shutter speed of about 1/100 sec, so as to be suitable for photography in bright daylight.
Recently, such film units have been known that have two differently sized stop apertures which are switched over depending upon subject brightness.
The photographic lens consisting of a limited number of small plastic lenses cannot sufficiently correct the curvature of the field. In order to reduce image deterioration that would be caused by the curvature of the field, the lens-fitted photo film unit holds the photo filmstrip to curve the film surface at the exposure position.
Also the photographic lens composed of a couple of lens elements has relatively large residual aberrations. Therefore, many kinds of refinements have been made on this type of photographic lens to improve image-forming performance. For example, Japanese Laid-open Patent Application No. 2002-23050 discloses a photographic lens consisting of, in the order from the object side, a first or front meniscus lens and a second or rear double-convex lens, wherein an image side surface of the first lens and an image side surface of the second lens are formed aspherical. According to this configuration, aberrations are corrected while improving the lens speed.
Indeed the photographic lens of the above mentioned prior art is designed to improve the performance, but if the first lens element or a front lens has an error of 0.01 mm in center thickness on the manufacture, variations in flange back will come to about 0.1 mm, that is about ten times the error amount. It is to be noted that the flange back means an ideal distance between the lens position and the film surface to be exposed in the camera. That is, the flange back is equal to the length from the lens to its back focus. Therefore, if the center thickness of the front lens has an error, the quality of photographs will be lowered remarkably.
In other words, the photographic lens of the above prior art is highly sensitive to the dimensional accuracy of its lens elements. In order to manufacture the lens elements with sufficient accuracy for achieving the designed performance of this prior art, the efficiency of manufacture would be lowered.
The above mentioned disadvantage of the prior art comes from the fact that the first lens tends to have a small radius of curvature on either side in the photographic lens wherein the first lens is a meniscus lens and the second lens is a double-convex lens, although this type of photographic lens is easy to correct the aberrations.
The larger the light bundle is refracted on the object side surface of the first lens, the larger the angle between the refracted light bundle and its optical axis becomes. So the height of the exiting light bundle from the first lens varies greatly as the center thickness of the first lens varies, resulting in bigger variations in the flange back.